The focus for DWDM transport systems has since the instruction in the mid 1990's been on increasing the bitrate per wavelength channel, the transmission distance and reducing costs. As the capacity per wavelength channel has steadily increased, the issue of spectral use has become increasingly important.
Firstly, this is due to the so-called ITU grid which divides the transmission spectrum into 100 GHZ or 50 GHz slots. Thus, if you want to upgrade a 10 Gbps channel on a 50 GHz slot to 100 Gbps, the 100 G signal needs to be much more spectral efficient per bit versus the 10 G channel.
Secondly, as the total traffic demand of the DWDM transport system increases, while new fibers are not deployed, the spectrum available for the growing traffic becomes limited, and overall spectral efficiency becomes an issue. A typical estimate for the annual backbone traffic increase is 30% per year, or 10 times in 10 years.
Thus, in the last couple of years, the field of elastic optical networks has been born. The idea is to allocate as little spectrum as possible to each traffic demand. The traffic demand is an element on the network traffic matrix between two end nodes. For demands having short transmission distances and few node hops, the needed spectrum can be made smaller by some modulation scheme with a higher value of bits/symbol (e.g., more bits per Hertz) at the same symbol rate, with less effect of filter narrowing from node cascades.
Tightly connected to any optical transport technology are the control plane and the ability to set up new connections in the network using signaling. In the IETF GMPLS framework, each connection, or path, is associated with a label, uniquely identifying the path in the network. In the IETF RFC6205, a WDM label is outlined, having information on the WDM grid and the carrier frequency of the wavelength channel.